The present invention relates to a method of isolating trioxane from an aqueous trioxane solution, which may contain formaldehyde, as further constituent by distillative separation techniques.
Trioxane has become more and more important, for example, as an intermediate product for the production of polyacetals. Trioxane is obtained by trimerizing formaldehyde, generally from an aqeuous formalin solution with a formaldehyde content of from about 30 to 70% by weight, which is heated up to boiling in a trioxane reactor in the presence of an acid catalyst. The so-formed trioxane is distilled off from the trioxane reactor in the form of a steam mixture substantially consisting of trioxane, water and formaldehyde. This steam mixture is the so-called synthesis steam. The acid catalyst remains in the reactor. The synthesis steam which includes about 12 to 16% by weight trioxane is normally fed into a rectifying column or tower connected to the trioxane reactor, whereby water and formaldehyde are concentrated in the tower sump, while a mixture with increased concentration in trioxane is obtained at the head of the tower. The trioxane-water-formaldehyde ratio in that mixture is about 35:48:17.
Pure trioxane cannot be isolated from mixtures with water or with water and formaldehyde by a simple distillation process at substantially atmospheric pressure, because trioxane forms azeotropic mixtures with water in the ratio of 70% by weight trioxane: 30% by weight water and with formaldehyde and water in the ratio of 65 to 60% by weight trioxane: 29 to 24% by weight water: 6 to 16% by weight formaldehyde.
To overcome the barrier to further trioxane purification due to formation of the azeotropic mixtures in the trioxane distillation, the trioxane containing mixture obtained in the head of the tower or rectifying column is normally subjected to an extraction treatment with an inert agent nonmiscible with water, for example, the hydrocarbon chlorides, methylene chloride or ethylene chloride, or benzene.
The disadvantage of the extraction treatment resides not only in that the separation of the trioxane from the extraction agent and its purification before reuse require additional operations, but also in that at working with organic solvents it cannot be avoided that the solvent would flow into the waste water and escape into the atmosphere, and such organic solvents, particularly hydrocarbon chlorides, especially when exposed to UV radiation, provide a strong environmental hazard.
It would thus be desirable to provide a method of isolating trioxane from an aqueous solution of trioxane, which may contain formaldehyde by a simple distillative method without an extraction step, at atmospheric or normal conditions of pressure.